1. Field
The present invention relates generally to magnetic memory and more specifically to tunneling magnetoresistive devices.
2. Related Art
Many advances in memory technology have been made in recent years. One such advance is the magnetic tunnel junction (MTJ) that makes use of the tunneling magnetoresistive effect to store binary information. MTJs are favored because they provide high data read speeds, are nonvolatile, and have a high magnetoresistive ratio. The basic structure of the MTJ is shown in FIG. 1 as a tri-layer device having a barrier layer 16 sandwiched between magnetic (ferromagnetic) layers 12 and 14. Each magnetic layer has an associated magnetization direction.
In a typical arrangement, one of the magnetic layers is configured to be a fixed layer 14. An anti-ferromagnetic layer (not shown) may be useful in fixing the magnetization direction of the fixed layer. In practice the fixed layer may alternatively be designed with a higher coercivity. Thus, the fixed layer is treated substantially like a permanent magnet with a permanent direction. The single headed arrow in layer 14 indicates that layer 14 is a fixed layer with a fixed magnetic direction. A second magnetic layer is termed a free layer 12. The free layer 12 is configured to switch the direction of its magnetization in response to a sufficient externally applied magnetic field. The double headed arrow at free layer 12 indicates that the magnetization direction of the free layer 12 may be inverted by an applied magnetic field.
In order to store binary data, the MTJ must have two possible logical states (i.e. binary states). These states are often referred to as “1” and “0.” The state of the MTJ is defined by whether the magnetization directions of the two magnetic layers 12 and 14 are parallel or anti-parallel. If the magnetization directions the two magnetic layers 12 and 14 are the same then they are said to be parallel. Alternatively, if the magnetization directions of the two magnetic layers are opposite, they are said to be anti-parallel.
The tunneling magnetoresistive effect is based on the phenomena that an applied magnetic field can influence the resistivity of a material. In simple terms, the resistance to a current passing through the MTJ is “high” when the magnetization directions of the magnetic layers are anti-parallel and “low” when the magnetization directions are parallel. Usually, the resistivity of an MTJ is determined by measuring a read current passed perpendicularly through each layer of the MTJ. A read current (i) is shown passing perpendicularly through the layers of FIG. 1. Because of the direction of read current flow, an MTJ is termed a current perpendicular to plane (CPP) device.
One measure for the quality of an MTJ is its magnetoresistive ratio defined as (ΔR/Rmin). ΔR is defined as the difference between the resistivity of the MTJ when the magnetization directions are anti-parallel and the resistivity of the MTJ when the magnetization directions are parallel, while Rmin is the resistivity of the MTJ when the magnetization directions are parallel (minimum resistivity).
In general, a current flowing through a wire produces a magnetic field. The direction of the magnetic field is perpendicular to the direction of current flow and follows the “right hand rule.” When writing to the MTJ, the magnetization direction of the free layer 12 is switched by applying a magnetic field to the MTJ. Usually, a pair of conducting lines running perpendicularly to one another are used to apply the external magnetic field to the MTJ by passing currents through each line. These lines may be termed a bit line and a digit line. The bit line may also be used to apply the read current.
The pseudo spin valve (PSV) is a second memory technology that uses the magnetoresistive effect to store data in a nonvolatile form. Unlike the MTJ, the PSV involves a conducting nonmagnetic layer (typically copper) sandwiched between two magnetic (ferromagnetic) layers. The basic structure of the PSV is shown in FIG. 2 as a tri-layer device having a conducting spacer layer 26 separating a magnetic sense layer 22 from a magnetic storage layer 24. Each of the two magnetic layers has an associated magnetization direction. The logical state of the PSV is determined by the magnetization direction of the reference layer.
In the PSV, the storage layer 24 is configured to switch its magnetization direction in response to the application of a magnetic field of at least a first threshold. Likewise, the sense layer 22 will switch its magnetization direction in response to the application of a magnetic field of at least a second threshold. The first threshold is generally higher than the second threshold—thus the magnetization direction of the sense layer 22 is easier to switch than the magnetization direction of the reference layer 24. Because the storage layer 24 has a higher switching threshold than the sense layer 22, the storage layer 24 is said to have a higher coercivity than sense layer 22. The storage layer 24 is shown in FIG. 2 as thicker than the sense layer 22 as an indication of higher coercivity. The double headed arrow at both sense layer 22 and storage layer 24 indicate that the respective magnetization directions of the layers may be inverted by an applied magnetic field.
Because the logical state of the PSV is determined by the magnetization direction of the reference layer, during a write sequence, a magnetic field of at least the second threshold must be applied to the PSV in order to switch its logical state. During a read sequence, a read current (i) is passed through the PSV. Typically, the majority of the read current (i) flows within the conducting spacer layer 26 because its electrical conductivity is greater than that of the sense and storage layers 22 and 24. Sense lines (not shown) may be coupled to opposing ends of the element for passing the read current (i) through the PSV. Because the read current flows parallel to the layers, a PSV is known as a current-in-plane (CIP) device.